1. Field of the Invention
The present invention relates to a spreading code synchronization method and others in mobile communication systems employing the direct sequence code division multiple access scheme for multiple access through use of spectrum spreading.
2. Related Background Art
The direct sequence code division multiple access (DS-CDMA) scheme enables a plurality of users to perform communications in an identical frequency band by transmission based on secondary modulation to spread the conventional information data modulated signal by a fast rate spreading code. Each user is identified by identifying a spreading code assigned to every user. For this reason, the receiver side needs to restore the received input signal of wide band to the original signal of narrow band in a despreading process, prior to execution of the conventional demodulation process. In the despreading process on the receiver side, correlation detection is carried out to detect correlation between the received signal and a spreading code replica synchronized in phase with a spreading code of the received signal. Particularly, an action to establish the phase synchronization between the spreading code replica of the receiver and the spreading code of the received signal at a start of communications, is called initial acquisition.
A common initial acquisition method of spreading code is a method of multiplying the received signal by the spreading code replica and integrating the resultant product over a predetermined time period, thereby determining a correlation between the two signals. In this initial acquisition method the resultant correlation output is then subjected to the square-law detection and whether or not synchronization is established is judged by determining whether or not the result of the square-law detection exceeds a threshold value. The correlation detection is generally classified under a case utilizing a sliding correlator to perform time integral and a case utilizing a matched filter has a function to perform space integral. The matched filter has a function equivalent to a plurality of sliding correlators arranged in parallel, in which the spreading code replica of each top is varied in accordance with the spreading code whereby correlation values over plural chips can be gained instantaneously. Accordingly, the matched filter operates very fast on one hand, but has large circuit scale and power consumption on the other hand, as compared with the sliding correlator.
The wideband DS-CDMA with the spreading band of not less than 5 MHz (hereinafter referred to as W-CDMA) is now under full commercial service. This W-CDMA is an asynchronous system where base stations operate based on independent time references. FIG. 1A is a diagram to show an example of assignment of spreading codes in the downlink in of an intercell synchronous system as a mobile communication system and FIG. 1B a diagram to show an example of assignment of spreading codes in the downlink in an intercell asynchronous system as a mobile communication system.
The cdma2000 scheme and the IS-95 scheme, which were proposed as candidates for IMT-2000 in the United States as the W-CDMA was, implement synchronization between base stations, using the GPS (Global Positioning System). In this intercell synchronous system (FIG. 1A), all the base stations have a common time reference. For this reason, the base stations can share a common spreading code with different delays for the respective stations. This intercell synchronous system can achieve the initial acquisition by performing synchronization of timing of the spread code.
In the intercell asynchronous system shown in FIG. 1B, the base stations have no common time reference and thus each base station is identified by a scrambling code. When a mobile station is powered up, it needs to establish synchronization with a scrambling code of a common control channel from a cell site to connect with a base station with the largest power of received signal (which will be called a cell or a cell site). This synchronizing method is called cell search in a sense of searching for a cell site for connection of a radio channel. In the intercell asynchronous system, the mobile station has to perform the cell search throughout all the scrambling codes determined in the system.
The known techniques of speeding up the cell search in the intercell asynchronous system include, for example, the spreading code synchronization methods described in EP 0825737 A1 and Japanese Patent Application Laid-Open No. H11-196460.
FIGS. 2A to 2D are diagrams to show an example of a transmitted signal in the cell search using mask symbols, wherein FIG. 2A shows a scrambling code before masked, FIG. 2B a masked scrambling code, FIG. 2C a common short code, and FIG. 2D a scrambling code group identification (ID) code. A channel with which a mobile station connects (radio-links) at the beginning of communications, is referred to as a perch channel. The perch channel is spread by the combination of the common short code 201 whose cycle period is a symbol period and which is common to all the base stations and the scrambling code 202 which is different among the base stations. Then the scrambling code is masked in a fixed period to produce symbols 2031 to 203M−1 spread by only the common short code 201. The common short code 201 is common to the base stations and the mobile station detects correlation values by means of the matched filter, using the common short code 201 as a spreading code replica. The mobile station can detect peaks at received timings of the signal spread by only the short code, regardless of the scrambling code 202. Timing synchronization of the scrambling code 202 is established by storing the received timings.
Further, scrambling codes 202 are preliminarily grouped. As receiving the scrambling code group ID code 2041 to 204M−1 representing a belonging group, the mobile station detects the scrambling code group ID code 2041 to 204M−1 representing the group, subsequent to the detection of the common short code. This can decrease the number of candidates of scrambling codes 202 to be searched. The scrambling code 202 assigned to each base station can be identified by detecting correlations at the received timings already obtained, using the product of the common short code 201 and the scrambling code 202 as a replica code, and performing the judgment with a threshold. By the three-step fast cell search method utilizing the scrambling code mask in this way, fast cell search is also implemented in the intercell asynchronous system.
The above-described three-step fast cell search method is modified as follows in the process of standardization of 3GPP (Third Generation Partnership Project). The common short code and the mask symbols spread by the short code and indicating the scrambling code group, are defined as Primary Synchronization Channel (PSCH) and Secondary Synchronization Channel (SSCH) independent of the perch channel, and the channel used for the identification of the scrambling code is also changed from the perch channel to a common pilot channel. The common short code is defined as PSC (Primary Synchronization Code), and the scrambling code group ID code as SSC (Secondary Synchronization Code).